Method of forming material



1943. G. D. RECHTON ET'AL 2,455,702

METHOD OF FORMING MATERIAL. oii inal Filed Sept. 7, 1942; v 2Sheets-Sheet 1 DELBERT J RD (440K WA 7 0 N ENTORS 2 Sheets-Sheet. 2

w m w w, llllllll Iv llllllllllllll I I k\\ MQJ 051.5537 J. WARD JACK H.WATJON BY a 4TTak/VEY APQ Wm www I G. D.RECHTON [-JAL METHOD OF FORMINGMATERIAL Original Filed Sept. '7, 1942 Dec. 7, 1948.

Patented Dec. 7, 1948 METHOD OF FORMING MATERIAL George D. Rechton, LosAngeles, Delbert J. Ward,

Sherman Oaks, and JackHJWatson, Los Angeles, Calif., assignors toDouglas AircraftCompany, Inc., Santa Monica, Calif.

Original application September 7, 1942, Serial No. 457,630, now PatentNo. 2,372,516, dated March 27, 1945. Divided and this applicationSeptember 20, 1944, Serial No. 555,028 r '1 Claims. ((1153-21) Thisinvention relates. to a method of forming bodies of metal andothermaterials, and more particularly to a method for forming. sheets ofmagnesium alloy and other metals having qualities which prevent theirbeing formed at ordinary temperatures without the development ofstructural weaknesses. The present application is a division of ourcopending application Serial No. 457,630, filed September 7, 1942, nowU.S. Patent No. 2,372,516. 2 l

l The word, form, as used herein means any processing operation whichestablishes changes the shape or dimension of an article or divides thearticle into a plurality of articles such as bending, punching bIanking,stamping, and shearing.

The aircraft industry, in line with the trend toward lighter and fasteraircraft, has been steadily increasing. the use of metals having lowspecific gravities such as magnesium. Although magnesium ranks. eighthin abundance among the elements of the earth, it has not been usedextensively in any industry until the last =few years. The slow adoptionof this element in industry has been due in a great degree todifficulties in perfecting economical and practical methods ofprocessing the same. 1

There have been many attempts to cold form sheets of industrialmagnesium alloys characterized by structural strength but theseattemptshave not met with any noteworthy success. For

example, it has been found that when it was attempted to dimple sheetsof magnesium alloys that incipient fractures would appear adjacent thesharp radius bends formed at the base of the dimple. These fracturesindicated structural weakness which made the sheet undesirable,particularly for use in aircraft construction.

Research and experimental work hasshown that by heating the sheets ofmagnesium alloys to an ascertained temperature, for example, in the caseof one magnesium alloy, to approximately 700 Farenheit, these metals canbe dimpled or otherwise formed without the occurrence of fractures.Following this discovery several methods were proposed to raise thetemperature of the sheet betfor-e the same was formed, but thesepreviously proposed methods, for several reasons, proved unsatisfactory.i

One method previously proposed consisted in heating the sheetelectrically by passing a current therethrough using the dimpling diesas electrodes in a manner similar to that followed in the spotweldingprocess. This method was unsatisfactory as it caused the dimpled metaland the dies used tobecome pitted from excessive arcing which took placebecause of the insulating qualities of thesurface protecting coat on themagnesium sheet. This difiiculty, coupled with excessiveheating andconsequentsoftening and deformation of the dies, rendered the machinefor this method of heating impractical and expensive to operate.

In order to avoid these undesirable results this invention contemplatesthe provision of a method in which there is employed a heating meanswhich does not involve excessive heating of the forming dies, and doesnot involve transmitting currents of electricity through the dies orother electrodes and the sheetof material being worked upon. Thepreferred form of the apparatus which can beused for carrying out themethod of the present invention embodies a means for heating outfracture, but less than the temperature at which the metal whensubjected to a minimum effectively suflicient flexural forming pressure,will be appreciably thinned or will be caused to flow appreciably. Withthe area heated to a temperature within this range the metal may besuccessfully formed if a pressure is used which is sufficient at theparticular temperature to which the metal is heated to form the metalbut not so high as to appreciably thin the metal or cause it to flow. li

In the preferred embodiment of the method of the present invention, theheating elements are first placed in close proximity to the area tobedimpled but held out of contact therewith until the area is preheated,after which the heat elements aremoved into contact with the oppositesurfaces of the sheet. After the area to be dimpled has reached thepredetermined ascertained temperature the area is formed or dimpled byforming tools. The heating element adjacent the area to receive themaximum pressure is held at a temperature higher than the other element.The area adjacent theheating element having the highest temperature-isnot excessively heated 3 for in the highest range of temperature thearea to be dimpled actually has a temperature only slightly in excess ofthat of the cooler element as heat is subtracted from the sheet by thecooler element. c Other features and advantages of the method 'of thepresent invention will be apparent from I operation, and a wiringdiagram of the eleetrical equipment for the heating elements ior controlof the pneumatic equipment and for performing other functions in theoperation of the machine to carry out the-method of the presentinvention.

While the method of the present invention may be carried out by anysuitable mechanism desired a dimpling machine has been selected toillustrate one machine which can be used to perform the method.

Figures 1 and 2 show a dimpling machine Ill having a base H upon whichismounted a supporting column I2. zontal laterially extending arms I3 andM are clamped to the column l2, the upper arm l3 supporting at its outerend a pneumatic tool operator i5 and the lower arm l4 supporting at itsouter end a lower dimpling'anvil assembly I6. I The tool operator H5 ismounted for vertical movement in the guide l8 at the outer end of theupper arm l3 by a lever H) which is 'iulcrumed at 2| between cars 22extending-upwardly from the upper arm l3 and which is connected at itsouter end to the upper end of the-tool operator at 2-3 and is connectedat its inner end to a piston rod E4 of a piston 25, shown only in Figure3, operating within a pneumatic cylinder 26 mounted upon the rear end ofthe upper arm 13. The lever 19 isspr-ing loaded by a spring 2-lcompressed between a stationary lug 23 and a collar nut 23 on a bolt 3iwhich passes through an eye in the lug '28 and is pivoted at its outerend to the free end of a depending arm 32 on the lever l9. As the sprin2] tends to move the left end of the lever I9 downwardly as viewed inFigure 1, a stop bar 33- is mounted on the arm I3 to engage the underside of the rear endof the lever 59 to prevent the piston from strikingthe bottom of the cylinder 26.

The lower end of the tool operator [5, carries a forming tool or femaledie 34 and is surrounded by a heating element or heating body 35 ofannular form' The heating element 35 is held in position laterally withrespect to the tool operator t5 and supported vertically by a piston 38which is vertically reciproc-ably movable within a pneumatic cylinder 31mounted upon the outer end of the upper arm [-3- adjacent the inner sideof the tool operator 15. Admission of air to the pneumatic cylinder 31moves the heating element downwardly and upone-Xhaust of air" from thiscylinder, the heating element is retracted to its normal position by-theaction of a tension spring 38 secured at its upper and lower ends to theguide H! and piston 36 respectively. A-cable 39 contains the electricalconductors for heating the element 35 and registeringits temperature.

To-the upper face of the outer endof the arm A pair of generally hori-I4 is secured a pneumatic cylinder 40 to which ail" is supplied at aninlet 4| in which reciprocates a tubular piston 42, schematically shownin Figure 3,, which carries atits'upper end'a heating element, orheating body 43 which is apertured for the accommodation of thestationary male dimpling die Hi. The support for the die I6 extendsdownwardly through the hollow piston 42 and is mounted upon the lowerarm [4. The heating element 43 is thus vertically slidable with respectto the stationary dimpling die I6 from a lower retracted position to anupper position in which it makes contact with the metal sheet to bedimnled. A cable 44 contains the conductor wires for supplying theenergy to heat the heating element 43 and to register its temperature.

microswitch 45 closed upon downward movement of the heater 43 to itslower retracted position, is connected to the control mechanism by acable 41, the cables 44 and 41 entering a junction box 48 from whichelectrical connections are made to. the control and indicating devicesby a cable 49.

The system of air conduits and pneumatic con trol devices for operationof the pneumatic cylinders 26, 3 1 and 40, and for operation of :thepneumatic cylinder 51 of the tool operator .15, are shownschematically'in Fig-ure 3, some of the conduits and control devicesappearing in the structural views, Figures 1 and 2. Air enters themachine under pressure at 52 through a gate valve 5'3 and may proceedto. an electrically controlled valve 54 through the pipjer55 and thence,whenvalve 54 is opened, through a pipe 58 to cylinder 26 to actuate thelever M te-depress the tool operator l5.

Air may also pass from the gate valve 53 to the electricallycontrolled-valve 51 through pipe 58, ior transmission when valve 51isinthe open position through supply conduit '59, and then pipes'w andelto cylinders 3 and 40 respectively for movement of the heating elements35 and 43respectively from their retracted position to their position ofcontact with the metal sheet to be dimpled. Electrically controlledexhaust valves 62 and 5'3 are in closed position when valves 54 and 51,respectively, are in open posi tion, and vice versa. The function torelieve the air pressure in cylinder 26' and in cylinders 31 and 40respectively for the elevation of the tool operator l5 in the case ofcylinder 25 and the retraction of the heatingelements 35 and 43 in thecase of the cylinders 3! and 40.

A valve 64 and a pressure gauge-61 in the supply conduit 59communicating with the T connection 66 joining the supply conduit 59with pipes and -6-l,provide means for regulating the air pressuretransmitted to cylinders!" and 40 at any desired value, thus regulatingthe degree of pressure at the heating elements engage the metal sheet.The section of exhaust pipe in which exhaust valve 62- is placed, theair supply pipe 55 and the pipe leading to the cylinder 26 areconnectedas shown bya T 6-8. The exhaust pipe in which valve 63 placed, the airsupply pipe 58' and the conduit 5% leading to the T' 55 are connected asshown by means of a T 68..

To T H interposed in the air supply pipe 5.5 is connected a pipe 12leading to. a pressure regune valve i3, which in conjunction with apressurel gauge 1:4. serves to. regulate the air pressure in pipe 15.leading to'the; cylinder Si or the tool operator l5 and tripping thepneumatic firing device for applying an impulse force to thedimplingcheBtwhen thepressure between the. upper and lower. dimplingdies reaches a prescribed value after the tool operator has been loweredand the two diesbrought into engagement with the metaljsheet to bediinpled, through depression of the tooloperator due to actuationoperating in cylinder 26.

Uponan anglesupport 18 secured to theupper arm I3 is mounteda controlbox 11 which houses the electrical control equipment for the supply ofcurrentto the heaters and 43 for sequential control of th'eelectricaldevices for the actuation of valves 54, 82, 51, and 63., Thesedevicesand their connections are enclosed within dash lines on Figure 3and designated by the numeral 18.

The valves .54, 82, 51, and 63 are electrically actuated by solenoids ina manner to be hereinafter explained, theconductors from the controlbox11 to' the various valve operatin solenoids pass ing througha connectionbox 19 mounted on the inner end of the upperarm I3. I

The front face of the controlbox 11 serves as a control panel 8I uponwhich are mounted the several dial switches and othermanualcontrols, andthevalue registering instruments for use in operating the machine. Theseseveral control devices and registering instruments will be referred toand describedin the course of the following. explanation of thewiringdiagramof Figure 3. On this diagram the arrows indicate the direction ofpotential dro Electrical energy issupplied to the control box by lead-incondu-ctors 82 and 83. Awarning light 84; connected across these maincondujctors 82 and 83, is placed on the panel toi ndicatepas sage ofcurrent to the control devices of the control box and thence to theheating elements and electrically operated valves. A line switch 85controls the supply of currentto the machine. Conductors 86 and 81constitute a shunt heating circuit connected to the wires 82 and 83 ofthe power circuit, and supply electrical energy through a transformer 88to the heating elements 35 and 43 in parallel through wires 89 and 9I.Th transformer 88 is of the variabletaptype and includes anadjustingknob 92. The transformer controls the wattage input andconsequently the temperature of the heaters 35 and 43. The primary orthe transformer. 88 isconnected to the conductors and81 by a normallyopen switch 93. This switch is automatically closed upon closing of themain switch 85 by a relay coil 94 which is connected between power lines82 and 83 by the conductor 81. and a conductor 95.

a The pressurevalve 51. and the exhaust 63 are operated by solenoids 96and 91 respectively,each valve being spring retracted to closed positionupon the deenergization of the associatedsolenoid. The solenoids 9B and91 are formed with a common terminal connected by lead 98 to theconductor 95 and the power line 82. The other terminal of solenoid 98 isconnected to theline of the piston 83 by a shunt circuit comprising theconductors I 99, I00, contact IOI, switch arm I02, movable betweencontact IOI and I03 conductors I04 and I05. The other endof the solenoid91 is connected to the power line 83 by ashunt circuit comprisingConductor. I06, contact I03, switch I arm I02, and conductors I04 andI05.

The pressure valve 54 and the n discharge alve 62 for operation of thepiston 25 are actuated by solenoids I01 and I08 respectively, each valvebeing sprin retracted to its closed position. Solenoid I01 is connectedto the power line 82 by the conductor I09 andlto powerline 83 by a shuntcircuit comprising theconductor III including the micro switch 45,conductor II2, contact H3,

switch arm H4 and conductors H5 and I05. Solenoid I08 is connected tothe power line 82 by the common conductor I09and to line 83 by a shuntcircuitcomprising conductor I I8, contact II0, switch arm II4, conductorH5 and conductor I05. I

A control circuit which is connected to the power lines 82 and 83comprises the conductor I I1, the switchI I8 operated by the foot pedalI I9 asbest seen in Figures 1 and 2, conductor I2I, and conductor I22 inwhich is disposed the relay coil I23. a

Another controlcircuit which is also closed by the foot operated switchI I8 and is arranged in parallel with the control relay circuit of therelay I23comprises conductors II1, I2I and I24, anode I25, and cathodeI26 of a rectifier tube I21, conductor I28, relay coil I29, anode I3Iand cathode the grid I38 to a predetermined value, the resistorI39limiting current flow from the grid I38to cathode I32. The grid ischarged with a positive voltage to the prescribed value of an R. C.voltage accumulator circuit I4I connected between the cathode I28 of therectifier tube I21 and the conductor I05. This circuit includes afiltering circuit I42 for filtering the same and thecircuit of the relayI29. As usual this accumulator cirg cuit comprises resistance andcondenser elements as shown. the time interval required for thevolt agebuild-rup being regulated by the variable re sistor I43. Discharge ofthe R. C. circuit is provided for by a discharge circuit comprising theconductor I 44 in which is interposed the usual resistance element I45having a contact terminal I45 adapted to be engaged by switch arm I41. jWhen the switch H8 is closed, the relay I23 is energized and movesswitch arm I02 into engagementwith contact I 0 I, connectingconductorsI00 and I04 to complete the circuit by which the solenoid 98 isenergized for the operation of the valve 51. g

Energization of relay coil I23 also moves the switch arm. I 41 out ofengagement with the contact terminal of the resistor I45 to openthedischarge circuit I44 thereupon the accumulator circuit I4I begins tocharge. When the accumulator circuit is charged with the predeterminedvoltage the grid I38 actuates the trigger tube I33 allowingcurrent toflow from the anode I3I to cathode l32 to energizethe relay coil I29.Energization of the relay coil I29 moves the switch arm I48 out ofengagement with the contact I49 whereby de-energizin relay coil I23.De-energization of the relay coil I23 causes the switch arm I02 to bemoved into engagement with the contact I03 to electrically connectconductor I08 with conductor I04 to complete the circuit forenergization of solenoid 91 for the operation of thevalve 33.Energization of the relay coil I29 also cau ses switch arm II4 to engagethe contact .II3 to electrically connect conductor I12 withconductor II5. and thus energize the solenoid I01 which opens the pressure valve 54for the loweri-ng'y Qfi lihfi. tool operatonl When. the. switch M8;is.=-opened the; relay I.2I1- is. deeeneizgizedtand the switch, arm. I48again. engages the contact M9,. Thede energizationlot the relay L29.also causes. the switch: arm. I, to, he, moved-intoengagement withcontact. -I-.I.0.- to connect conductor H6 with the conductor I I5 andthus complete the circuit for energization. otthe solenoid, M8 foropening; the, dischar e. valve 62, resulting; in..the elevation.Qf..the.-t,ool: operator. I 5.;.

The. microswitch: 45. is inclosed position, when. theheater element, 43.isin its lowermost. position and opens at the initiation of the upward.movement. of, this heater in. response to. opening of; thevalye 62;.Ther-micnoswitclt -is, interposed in. the conductor III and prevents,operation. of the. solenoid m l. in response to. closing. of theswitch-Hi3, tld until theheater. 4.3.has reached the, lower limitofitstravel.

A switch= operator, whichis shown: onthe --panel M at L53 operates. anormally closed-switch I54 interposed in. conductor. llmand anormallyclosed switch. I55: which. normally. shortcircuits a.high resistanceelement I55 of the voltage accumulator circuit. MI. The operator I53.in. one. position closestheswitches I54 and. I55 for normaloperation ofthe machine. When the operator: I53, is thrownv toits other. position,switches. I 5.41 and. L55 are. opened, having. the. effect of preventingmovement. otthe tool. operator L5. andincreasing thetime-intervalrequired to, charge. the grid. I38: sufficiently for.passage. of current. fromanode. I35. tocathode I32. ofthe-gas. trigger.tube I33.

The. machine. as, disclosed in. our said. copending application, is alsodesigned. for dimpling I Dur.al." or other metals. which may be. workedcold. without the, developmentot structural weak.- ness- Aswitchoperatormountedon the-panel.8.l., and indicated. in. figure. Z by-the. numeral.I51, operates. four. switches which. appear. on. the. wining diagram of.Figure 3. and aredesignatedby the numerals I58} I59, LB'I', and-I52.Switch-I58 is interposed. in, conductor 95, andwhen open preventsenergization of. the relay; coil. 9A whichv controls the heater,circuit. Switch I.5.9.has,two,,.positions, designated. on the diagramas:,M. and D. When this switch is in position M conductor H5 isconnected. to conductor, I05. permitting. operation of thepressure valve51 and; dischargevalve 63-. When it is:in positionD itshortecircuitsthevoltage accumulator discharge circuit/[Mirannecting this circuit throughconductor Ifiaito conductor I05:

Switch IBI has two positions, indicated by the letters M and D: When inthe position Mt conductors 99" and Illfi are connected completing thecircuit for operation of the valve 5?. 'When in position D' conductor III is connectedito con.- ductor; llll'l'through a crossecircuitconductor. I64 for energization' of" solenoid I01 for the operationofpressure valve. to lower. the tool, operator.

Switch I62 has .two positions, designatedby the letters-M andD. Whenthis-.switch is. imposition M' the circuit for the-solenoid?! is,complete and the discharge valve';63* may, be operated. to retract'theheating elementsitotheir:normal p0,: sitions. When t'hisswitch isinjpositionD' itlconnects; conductor I! 6 with. conductor. I06 throughthe'crosscircuit conductor. I65, completing, a Cir.- cuit for theoperation of'solenoid' H18; which ac.- t-uates-discharge valve swarmselevation otthe tool operator.

When the switch operator lli'Lis turnedltothe lefthandposition, as shownin-.Figure Z andde's ignated Dow; switch I58 is" closed andiswitches war8 5.9;; LIiL. and. Lil are. in theinM-position When the, switchoperator. I51 isin the. right-hand position, designated.Dural, switchI5Iiis openand'the other three switches are in. their. Dposition. n

he dimpling. dies. may. be. any, form. desired and the heatihg elements,asabovedescribed; are preferably annular in form ioren'circlement ofthe, dies As disclosed. mour previously filed application Serial No.4573330, now. UQShPatent No. 2;372,5l6, .the work. contacting. faces. ofthe. ele-' mentsare preferably iormed of. pur silver. The silver. hasthe combination. of high. specific, heat, i h conductivity andnon-oxidizing qualities, whi'oh..makes it extremely practical as acontact facing. The. lipid pressure. for, the actuation of the tooloperator I5 should be closely regulated. by valve I3, in correlationwith the regulation ofthe temperature of the heating elements. Excessivepessure at a given. temperature of? the metal sheet may causethinningofthedimpl'ed and adjacent. areas or other deformation .of'the,sheet. Insufficient pressure at a giventemperature may result inincompletely formedv dimples. This relationship of pressure andtemperature may have. the followingtheoretical explanation: Magnesium.and its. alloys, particularly aluminum and; zinc. containing alloys, do.not readily lend. themselves to coldv forming. It is well known that thehexagonal magnesium crystals exhibit fewer slippage planes than crystalsof metals which crystalize in more highly symmetrical, forms. Attemperatures below 410.". F. slippage occurs only onthebasali planes;while at temperatures. above 410 F. pyramidal slippage. occurs. andplasticity increases; very markedly.

It..is,, therefore,,possib1e to hot work magnee. sium andits alloys ifthe workpi'eceis heated to atemperature aboyejthat temperature at whichpyramidal slippage comes into operation but less, than that temperatureat. which-the. metal will be. thinned or will be caused to flow by aminimum efiectively sufficiently fiexural. forming; pressure. Thecritical. slippage point, that is,, that temperature below which. thesemetals willnotyield under pressure along bothpyramie dal and basalplaneswithout fracture. will. of course vary with alloys of. different.magnesium content.

Forming the metalsheet at. av lower tempera.- ture and, at acorrespondingly higher, pressure has an additional. advantage for.certain.job...re.- quirements, Highenformingpressures of apercussive.character have awork hardening. effect on. the metal. Where.thisworlehar'dening is..an advantage. in the finished product,,.it may,beaceompli'shed with metals, suchas the'magnesiiun alloys hereindescribed} by adiustingjthe per cussive pressure ofjthe forming tool. toa value which'is suificient to work harden the. formed zone andheating,the metaljto. a corresponding temperature between points a. and c. l

The; heating elements are designed to have different. temperatures, the.lower heating element 43. being maintained'ifor example at a temperaturepreferably of; about 6.50" F. and the upperheater..35Ibeingmaintained1at a tempera.- ture of preferably 900'?v E.for. dimpling, the hard rolled; magnesium alloy known. as Dow metal J,the composition of'which. is 92.5% magnesium, 6.5% aluminum andtheJremainder of zinc. and manganese. The lower heating element. in it'snormal retracted position is spaced relatively close to thesheet ofmagnesium metal and. the

- the metal sheet.

upper heater is spaced relatively distant therefrom. The heating face ofthe lower element 43 is also preferably greater in area than the heatingface of the upper heating element 35. fhe capacity of each heater issufiicient to maintain l temperatureof the. lower heater. This is truebecause, as has been explained above, thisheater is normally maintainedat a temperature less than the upperheater and as it absorbs heat fromthe sheet the temperature thereof will more its heating surface at itsprescribedtemperature.

As a result of these relative heating capacities, heating surface areas,and spacings from the metal sheet, the two'heati ng elements, while heldin their retractedppsition, will, by radiation and to some extent byconvection ofair currents, heat the metal sheet, although to a degreesubstantially less than 650 F., theupper heating element contributingto. a substantially lesser extent to this heating action.

As the heating elements are moved intoand out of contact with the sheetto successively dimple a row of rivet holes, the temperature of the areasurrounding. any given rivet hole will rise as the operator moves ittoward the dimpling dies, reaching a temperature in the ex-' ample abovestated of 650? F., or nearly that temperature, when it is between thedimpling dies.

When at this juncture the heating elements are moved into contact withthe metal sheet, the upper heating element acts to bring. the

temperature of the metal sheet rapidly to the optimum workingtemperature in a relatively few seconds. In the operation of the machineto carry out the method of the presentinvention, after a metal sheet isinitially heated prior to the first dimpling operation thereon, it re-.quires as little astwo seconds of contact of} the heating elements withthe sheet for bringing proper Workingtemperature. Theupper heater willnot overheat the metal in this brief time interval, since the lowerheater, being heated to a temperature less than that of the upper heaterand being of larger size and having a silver facing, will absorb heatfrornthe upper ation is that of dimplingand the adjustment pro- .videstoo high a temperature, the dimples will be indented too deeply and thesheet will warp.

. If the temperature is too low, the sheet will develop cracks aroundthe dimple. I

t I A thermocouple (not shown) is embeddedlin the silver facing of eachheating element. Referring to the wiring diagram of Figure 3, the

.thermocouple of the upper heating element. is connected by conductorsI69 and HI to the terconductors I and I16 to an electric temperawiththeterminals of the conductors I13;and I14,

since it is more important in the method therein closely approximate thetemperature of the sheet. Whenthe operator desires tocheckthetemperature of the upper heater he may do so by throwingthe.switch..l12 to connect concluctorsJBB and m m with conductors I15and m leading to the The heating elements and 43 are so constructed andsecured in place that they are readily :removable, lthe flexiblec ableleading thereto be- ]{5 ing provided, as shown in Figure 1, for that prposeflwith jacks atJI1B and n9. Thisfeature of construction of themachineis of advantage in the method herein disclosed for it makes itpossible to change the heating elements, usingfor 2 metal sheets ofdiflerentfthiclmess and of differ icharacterfl t a t t f The control box11, as shown in FigureZ, car- "ries the operator I8I for the switch I62for selecent curvature, heating elements of corresponding -35 tivelyconnecting the thermocouples of the heaters with the temperature meterI11, which'is also mounted in the control box. The lamp 84 is mounted onthe panel 8|, which also carries the operating lever I83 ofjthe lineswitch 85,the

manipulating knob I84 of the variable resistor that portion of itsurroundinga rivet hole to I.43,,.as Well as the switch operators I53and I51.

In carryingbut the method of the present irfvention .On the machineherein. disclosed for dimpling magnesium alloy, the knob I51 is turned.35 to the Dow. position and the operator I83 manipulated to close thelineswitch 85. jjClosing of theline switch 85 lights the beaconlamp 84and energizes the heating circuit relay 94 which close switch. 93,topass current through the 4n heaters,43 and 45. The temperature of theheaters isf regulated by adjustment of the transformer 88. throughmanipulation of the transformer manipulating knob SZ. Since the twoheaters are connected in parallel, the temperature of the heaters areproportionatelyraised or lowered together. However, the heaters may beconnected, so that the temperature of the one heater mayberegulatedwithout affecting the temperature of the other heater.

5Q .fAsheet .of. magnesium alloy is now placedin the machine with one ofthe rivet holes therein placedupon the usual pilot of the lower die,radijationfrom the lower heating element, and to a lesser extent fromthe upper heating element,

causinga substantial elevation of the temperature of the metal sheet.Closing of the line switch 83 also sends current through filament I35 ofthe rectifier tube I21. If, as is usually thecase, it is desired, topreheat the metal before dimpling 6O the first rivet hole in the sheet,switch operator 1 IE3 is temporarily thrown at this time,

switches. I 54 and I55.

o e in Upon depression of the foot lever I I9 to close the switch II8,current flowsto the control relay 123 which connects the circuit leadingto the .minals on one side of a double-throw switch I12. pressufg v lv51 and opens the switch in dish thermocouple f th w r t r is o chargecircuitlM. Current now flows through notified y. conductors 3 and toiflconductor 98 to the solenoid 96. actuating presjminals on theotherSid fw i ch, The sure valve 51 operating the piston in cylinder movable,arms of this switch are connected by 37 and the piston in cylinder II]to bring the end of a time interval, which may be regulated disclosedthat the operator have indicated the for example attwo seconds byadjustment of the knob 181, the .gas trigger tube I33 .is conditionedior passage of direct current from the rectifier tube J21 through therelay .coilII'ZB. When this .coilis energized switches vI 4B and 14.9are opened de-energizing coil [23, switch arm H4 engages contact I13connecting conductors H2 and ,I I5, and disconnecting conductors I16 andiii). De- -en.eiig'ization of relay coil {I23 .conn ects conductors.ITDQB and H14 to energize solenoid 911,.actuating discharge valve 63 towithdraw the heating elements totheir retracted position.

FDe-energization-of coil 123 also returns switch arm Into the terminalcontact I '46 of the resistor I415 permitting discharge of the R. C.circuit III-I through the discharge circuit I44 resetting this circuit.forthe next energization of relay coil I29. The ,gas trigger tubeI'3f3, as is-evident from the above, allows energization of relay coilI29 to hold the heating elements in contact with the .metal sheetfor aprescribed time interval which is unaiiected reactively by the movementsof the pneumaticallyenergized devices or the flow of current through thesolenoids which operate these valves. This electronic control of theheat- .ti-me interval has a definite advantage .over .a mechanical,pneumatic .or other type of time control. .It ishighly accurate and isunaffected by the vibration or jars which unavoidably attend .theoperation of the machine.

When the ,lower .heating element closes the microswitch 45, and assumingthat the switch as controlled by operator ,-I'5.'3 have been opened, thecircuit which energizes solenoid ID! is com- :pleted except for theswitch I54 which is held open by the .operator I53. Since the .operatoralsoisholdingswitch I55 open, the time interval .ior conditioning thegas trigger tube I33 for passage of current from its anode 131 to its.cathode 1'32..for the energization of control relay coil (IE .islengthened by several seconds, for example 5.seconds. Therefore, aslongas the operator 1.53 holds switch 154 and switch I'5'5 in open position,the dimpling gnncassembly will not operate and the heating phase of theoperative cycle will .be lengthened.

When the sheet is brought close to dimpling temperature, the operator153 is moved to close switches I54 and 155, shortening the time intervalof .contact of the "heating elements with the metal :sheet and closingthe circuits to the dimpling gunsolenoids 'I'Il'l and 1.08. When thecon- .trol relay coil 1 29 ,is energized "by operation of the gastrigger tube 133, the lower heating element 43 will close themicroswitchl5 completing the circuit to solenoid I01 for the actuation of pressurevalve 54. .to lower the tool operator. When .the too'loperator comesinto contact with .the .meta1..sheet,,ii .it.isoi the one-shot type, itwilldeliver its percussion blow to the upper dimpling die upondevelopment of the prescribed pressure against the metalsheet and if itis of the rattle type it will deliver a succession of iblowsin themanner characteristic of percussion toolsiof this kind.

.The operator now .lifts his foot from the foot lever M9,, openingswitch II8 which breaks the circuit to relay I29. 'De-energization ofrelay I29 closes the circuit .to solenoid 'IUBWhich then ac- .tu-ates.the valve 62 to permit-the spring 21*to elevate the tool operator I5 toits retracted position. .De-energization of relay I29 also, as:described above, closesswitches I48 and llawhich resets the circuit torelay I23 for its energization, when the operator shall again close footoperated switch .I'Ii8. In using the machine-the operator releases theToot pressure on "the pedal 12 -I I9 .to raise the tool operator at thecompletion of each .dimpling operation and again depresses pedal I49after he has moved the metal sheet to bring the next rivet hole intoposition for the next dimpling operation, whereupon the cycle isrepeated.

While the value of the fluid pressure causing movement of the heatingelements into contact with the .metal sheet may vary substantiallywithout disadvantage, it should not be so great as .to injure the metalsheetnor so small that the heating faces of the elements will notcontactthe metal sheet with sufficient firmness to provide freeconduction of heat from the heating element to the metal.

If desired, the machine may be adjusted to hold the heating elementsstationary in their retracted position and the heat within the dieswhich is received from the encircling heating elements relied upon toraise the metal sheet to proper forming temperature at the time the diescontact the sheet when brought into 'forming position. To successfullycarry out this method the heating elements must be maintained at ahigher temperature for any given forming ,prcssure than theirtemperature when used in the contact method above described and also thedies must "be made of a material which will not lose hardness or otherqualities desirable in die forming work, at'the temperature required tothus bring the metal sheet to forming temperature. Preferably thematerial of the die should be relatively non-oxidizable at such atemperature and of high specific heat and high thermal conductivity.

The expression heating element or heating body as used herein is not tobe limited to an electrically heated element or body, except wheremodifying or amplifying context implies such a limitation. Theexpression defines a body or member which'has oris associated'with heatdelivering means whereby its temperature may be raised, whether thatmeans be an electric current conductor, a heated fluid like steam, orother means.

While there have been described herein certain embodiments of thisinvention, the invention is not to be understood 'as confined to theseparticular embodiments, for 'it is susceptible to changes in form anddetail within the scope of the following claims.

We claim:

1. A method of dimp'ling between pressurally related forming elementssheets of a material which cannot be satisfactorily formed below itscritical slippage point which comprises: radiationally preheating apreselected local portion of a sheet of said material by mounting thesheet between two heating elements held at difierent degrees of heatwith the portion to be dlmpled in sufliciently close proximity to theelements as to receive radiant heat therefrom; moving said elements intoengagement with the opposite surfaces oi I the sheet at the localportion thereof to conductively "heat said portion; removing saidelements from contact with the opposite surfaces -oi' th'e sheet; andthereafter applying sufficient percussive ilexural force at the :centerof the heatedl'ocail portion to properly flexura'lly form the materialwithout appreciably thinning the same car-causing it to appreciablyflow.

2. A method of dimpling between pressure applying elemsnts sheets of amaterial which cannot be satisfactorily 'f-ormed 'below its criticalslippage point which comprises: pro-heating a.

tween two heating element'sof annular form held at different degrees ofheat ,withthe portion of the sheet to be dimpled in close proximity tothe element'of the lower temperature; moving said elements intoengagement with'the oppositesun faces of said portion of the sheet to bedimpled; holdingfsaid elements injengagement with said opposite surfacesuntilthe portibnof the sheet to be formed is heated to a temperaturebetween the critical slippage point of the material and the degree ofheat at which the material will flow even when subjected to a minimumeffectively sufficient forming pressure; withdrawing said elements fromcontact with said surfaces of the sheet and moving said pressureapplying elements coaxially of one of said heating elements intopressural engagement with said heated portion; and thereafter applyingsufficient pressure to said heated portion to properly flexurally formthe material thereof without appreciably thinning the same or causing itto appreciably flow.

3. A method of dimpling between pressure applying elements sheets of amaterial which cannot be satisfactorily formed below its criticalslippage point which comprises: pre-heating a preselected local portionof a sheet of said material to be dimpled by mounting the sheet betweentwo heating elements held at different degrees of heat with the portionto be dimpled in close proximity to the element of the lowertemperature; moving said elements into engagement with the oppositesurfaces of the sheet with the elements substantially surrounding thelocal portion of the sheet to be formed; holding said elements inengagement with opposite surfaces of said sheet until the coolerelementreaches a temperature greater than the critical slippage temperature ofthe material but not greater than the temperature at which the materialwill flow when subjected to a minimum effectively sufficient flexuralforming pressure; and thereafter applying suflicient percussive forcesubstantially to the center of the portion heated to flexurally form andharden the material of said portion without causing it to appreciablyflow.

4. A method of dimpling sheets of material which has a critical slippagepoint, comprising:

applying heat to opposite surfaces of a preselected local portion of asheet of said material to quickly raise the temperature thereof;applying flexurally forming pressure to said surfaces; increasing themagnitude of said fiexurally forming pressure while continuing saidapplication of heat and so correlating said heat to the melting point ofsaid material as to inhibit warping and burning of said material; andsuddenly triggering upon said heated portion a dimpling force of amagnitude predeterminedly so correlated to the critical slippage andflow points of the material of said sheet, and to the then existingtemperature of the material of said portion, as to effect sharplydefined dimpling of the same, free from fractures, without appreciablythinning or flowing said material.

5. A method of dimpling sheets of a material which has a criticalslippage point, comprising: pre-heating a preselected local portion of asheet of said material by placing said portion between 'imity to thelowermost heating means, the temv 17:4 ing pressures while continuingtheheating of said material .and so correlating the temperature of "saidmaterial to the melting pbintthereof as to inhibit warping and burning'ofj saidmaterial; and suddenly triggeringupon said heated portion adimpling force of a magnitude predeterminedly so correlated to thecritical slippage and flow point offlthe material of Saidsheet, a'ridtdthe then existing temperature of the material of said portion, to forma sharply defined dimplej free from fractures; without appreciablythinning or flowing said material.

6. A method of dimpling sheets of a material which has a criticalslippage point, comprising: pre-heating a preselected local portion of asheet of said material by placing said portion between a pair of spacedheated bodies; moving said heated bodies into engagement with oppositesurfaces of correlated to the critical slippage point and flow point ofthe material of said sheet, and to thenexisting temperature of thematerial in said minute portion, as to effect sharply-defined dimplingof same, free from fractures, without appreciably thinning or flowingsaid material,

7. A method of dimpling sheets of a material which has a criticalslippage point and is readily combustible and warpable, comprising:pre-heating apreselected local portion of said sheet by placing the samebetween superimposed heat ing means with the lower surface in closeproxperature of which is less than the temperature of the uppermostmeans; applying said heating means to opposite surfaces of thepreselected local portion thereby transferring heat from the upperheating means through the sheet into the lower and cooler heating meansat a rate sufficient to quickly raise the temperature of the preselectedlocal portion while inhibiting rise in the temperature of the sheetabove the temperature of the lower heating means; applyingflexurallyforming means to opposite surfaces of the sheet at saidportion; increasing the pressure of said forming means on said sheetuntil the preselected local portion is raisedto a temperature betweenthat at which fractures occur in said material when the same is workedand that at which the material will thin or will flow when subjected tothe minimumeifective forming pressure; withdrawing said heating meansfrom engagement with said preselected local portion; and suddenlycausing said forming means to apply a predetermined percussive force tosaid preselected local portion of a magnitude and time of application socorrelated to the critical slippage pointand flow point of the material,and to the then existing temperature of said preselected local portion,as to flexurally sharply dimple the portion free from fracture whileleaving the portion in an unflowed condition.

GEORGE D. RECHTON. DELBERT J. WARD. JACK H. WATSON.

(References on following page) mwrmmoz :file ROf *this patent:

UNITED sums Number Name .Date Schmidt et a1. Dec. 15, 1936 VonTannenb.erg ..1u1y 1, 1941 Lundgmnet-a1.. Sapt..:30, L941 Mei-tJune-30.,.l942

FOREIGN PATENTS Number Country Date 511,106 Gne alt Britain June, 1942OTHER REFERENCES P. 25 of vAeronautical Sciences, v.01. 8,1810. 1, Nov.1940.

Metals Hand-book, 1939 ed .pububy AmerFEMSoc. m for Metals,7016111101111 Ave., Clevland, Ohio.

